The present invention relates to a method of making submicron cemented carbide with extremely narrow grain size distribution.
Cemented carbide inserts with a grain refined structure are today used to a great extent for machining of steel, stainless steels and heat resistant alloys in applications with high demands on both toughness and wear resistance. Another important application is in micro drills for the machining of printed circuit board so called PCB-drills.
Common grain growth inhibitors include vanadium, chromium, tantalum, niobium and/or titanium or compounds involving these elements. When added, generally as carbides, they limit grain growth during sintering, but they also have undesirable side effects, affecting the toughness behavior in an unfavorable direction. Additions of vanadium or chromium are particularly detrimental and have to be kept on a very low level in order to limit their negative influence on the sintering behavior. Both vanadium and chromium reduce the sintering activity often resulting in an uneven binder phase distribution and toughness reducing defects in the sintered structure. Large additions are also known to result in precipitation of embrittling phases in the WC/Co grain boundaries. According to WO 99/13120, the amount of grain growth inhibitors can be reduced if a carbon content of the cemented carbide close to eta-phase formation is chosen.
Grain growth inhibitors limit the grain growth during sintering. However, since they generally are introduced in powder form their distribution is not as even as desirable. As a result in the sintered structure there often appear areas with abnormal grains of WC. A solution to this problem is disclosed in U.S. Pat. No. 5,993,730 according to which the WC grains are coated with Cr prior to the mixing operation. In this way the number of areas with abnormal grain growth can be reduced. However, larger grains from the original powder still remain in the sintered structure. The grains result from grain growth during the carburization operation. A solution to the problem is disclosed in JP-A-10-212165 in which tungsten oxide powder is mixed with powder of chromium oxide or chromium metal, reduced in hydrogen mixed with carbon powder and carburized to WC. Again because of the uneven distribution of the chromium a certain grain growth during carburization can not be avoided.